Standard mosaic geometries in modern imaging systems, such as a thermal imager, give rise to the fundamental limitation of creating undersampled images, which are degraded by aliasing. This problem is considered to be the most important effect which limits recognition and identification ranges for these systems in various surveillance scenarios.
Aliasing is a distortion affecting spatial frequency components of the image that are higher than half of the sampling frequency. A point target image can either cover one detector element and generate a strong single signal or cover up to four detector elements which will produce much weaker signals from all four elements. Variable registration of the image's continuous signal with the sampling lattice leads to small output signal fluctuations resulting in all edges of a spread target being degraded by the spurious registration.
Two known techniques have been used to, at least partly, solve the aliasing problem. A first solution is called information prefiltering and simply consists in eliminating all frequencies that are higher than half of the sampling frequency. This is realized in imaging systems by matching the blur circle of the optics to an array of at least 2.times.2 detectors, i.e. each point of the image on the mosaic covers at least 2.times.2 detectors. This prefiltering of the information provides a good solution to undersampling problems but is, however, extremely expensive to implement since an array of at least 1024.times.1024 detector elements will be required to obtain a 512.times.512 image resolution which is about typical for T.V. An array of 1024.times.1024 detector elements is practically impossible to achieve at present with infrared focal plane arrays of high sensitivity and any array with that number of detectors elements will be extremely expensive to manufacture.
A second solution to reduce the effects of aliasing is to introduce a dithering or microscan into the system. Introducing a scan mirror in the optical path allows multiple images to be formed on the mosaic of detector elements with a small displacement between each of the images. A single plane mirror actuated by piezo-ceramic transducers may be used as a microscan mechanism such as that described in an article by R. J. Dann et al on pages 123 to 128 of SPIE, Vol. 685, Infrared Technology XII (1986). This mirror causes an image scene to be displaced by some fraction of a pixel with respect to the detector array so that interpixel sampling can occur in both horizontal and vertical directions. For instance if a 2.times.2 microscan is applied, the first field records the image at a first reference position on array. The image is then displaced by half a pixel to the right to record a second image and then a half a pixel vertically to record a third image. The image is then displaced a half a pixel to the left for recording a fourth image and then vertically to return the image to its original position. These microscan images are electronically merged together as interlaced fields to form a regular full frame image containing the 4 fields for a 2.times.2 micro scan as previously described. In a similar manner, the image could be displaced by one third of a pixel for each step to implement a 3.times.3 microscan.
A number of problems are associated with the use of microscan image such as significant sensitivity reduction caused by the use of many fields to build up an image. This severely limits the duration over which the incident radiation power may be integrated resulting in less sensitivity. The imagery produced by microscan systems is also interlaced which is less suitable for automatic recognition systems and other auto-processors used in modern surveillance systems. Furthermore, a prime objective of developing staring systems is the elimination of scanning mechanism in an IR camera to allow smaller, lighter, cheaper and more reliable imaging systems. The introduction of the microscan mirror can generally be considered as a retrograde step that moves in the opposite direction desired by system designers in that more electronics, more power, more volume and sophisticated mechanics are required to control that type of system.
A hexagonal, rather than square image elements, mesh imaging system has been extensively studied for reasons other than aliasing control. However, the hexagonal mesh cannot easily be addressed by either Cartesian coordinated or polar coordinates. It is, therefore, laborious to use hexagonal meshes with all memory buffers and most image processing algorithms since they rely on Cartesian coordinate systems. Furthermore, the size of detector elements in a hexagonal mesh is less than that for square detector element resulting in a loss of sensitivity.